An insulation coating of a magnetic steel sheet used for motors, transformers and the like is required to have various properties such as not only interlaminar resistance, but also convenience in working, storage, stability in use and so on. Since the magnetic steel sheet is used in a variety of applications, it is attempted to develop various insulation coatings depending upon the application. When the magnetic steel sheet is subjected to punching, shearing, bending or the like, magnetic properties are deteriorated due to residual strain so that stress relief annealing at a temperature of about 700 to 800° C. is frequently conducted to solve this problem. In that case, therefore, the insulation coating must be resistant to the stress relief annealing.
The insulation coatings are roughly classified into three types:                (1) an inorganic coating placing great importance on weldability and heat resistance, and being resistant to stress relief annealing;        (2) a resin-containing inorganic coating aiming to establish punchability and weldability, and being resistant to stress relief annealing (i.e., semi-organic coating); and        (3) an organic coating that is unable to be subjected to stress relief annealing in a special application, but the coatings containing an inorganic component as shown in the above items (1) and (2) are resistant to the stress relief annealing as a general-purpose product, and they contain a chromium compound.        
Particularly, chromate-based insulation coating of type (2) can dramatically improve the punchability in a 1-coat and 1-bake production as compared to the inorganic insulation coating and are widely used.
For example, in JP-B-S60-36476 is disclosed a magnetic steel sheet having an electrical insulation coating obtained by applying a treating solution formed by compounding a dichromate-based aqueous solution containing at least one divalent metal with 5-120 parts by weight as a solid content of a resin emulsion having a ratio of vinyl acetate/VeoVA as an organic resin of 90/10 to 40/60 and 10-60 parts by weight of an organic reducing agent based on 100 parts by weight of CrO3 in the aqueous solution, onto a surface of a base iron sheet and then subjecting to baking according to the usual manner.
However, environmental awareness is rising in recent years. Hence, products having an insulation coating free of chromium compound are demanded by consumers even in the field of magnetic steel sheets.
There are developed magnetic steel sheets with an insulation coating containing no chromium compound. For example, as an insulation coating containing no chromium, but having a good punchability are described ones containing a resin and colloidal silica (alumina-containing silica) in JP-A-H10-130858. Also, an insulation coating comprising one or more of colloidal silica, alumina sol and zirconia sol and containing a water soluble or emulsion resin is described in JP-A-H10-46350, and an insulation coating composed mainly of a phosphate containing no chromium and containing a resin is described in Japanese Patent No. 2944849.
However, these magnetic steel sheets with the insulation coatings containing no chromium have problems that the bond between inorganic substances is relatively weak and the corrosion resistance is poor as compared to those containing a chromium compound. Also, when back tension is applied by rubbing the steel sheet surface with a felt in slit working (by using a tension pad), the occurrence of powdering comes into question. Furthermore, there is a problem that the coating becomes weaker after stress relief annealing and is easily scratched.
For instance, the above problems can not be solved by simply using one or more of colloidal silica, alumina sol and zirconia sol in the method of JP '350. Also, sufficient investigations are not made with respect to the case of mixing these components in combination and at a specific ratio. Furthermore, in the case of a phosphate coating with a composition containing no chromium as described in JP '849, sticking occurs and the water resistance tends to be deteriorated.
These problems are apt to be easily caused during baking at a relatively low temperature of not higher than 300° C., and the occurrence becomes particularly remarkable at a temperature of not higher than 200° C. Meanwhile, the baking temperature should be as low as possible from a viewpoint of the reduction of energy consumption and production cost, and so on.
In addition, when using the methods described in JP-A-2007-197820 and JP-A-2007-197824, i.e., when using a coating comprising a polysiloxane polymer obtained by copolymerizing polysiloxane with various organic resins and an inorganic compound such as silica, silicate or the like, there are problems that blowholes occur in the TIG welding, and that a spotted pattern occurs after annealing depending on the kind of the steel.
We discovered that, by compositely including a Zr compound and an Si compound containing a plate-like silica, or further a B compound as an inorganic component in the semi-organic coating, a number of the issues raised above can be addressed.
We thus provide:                1. A magnetic steel sheet provided on its surface with a semi-organic insulation coating comprising an inorganic component and an organic resin, characterized in that the inorganic component of the coating comprises a Zr compound and an Si compound containing a plate-like silica as a ratio in a dry coating of 20-70 mass % of Zr compound (converted to ZrO2) and 10-50 mass % of Si compound containing a plate-like silica (converted to SiO2), and the remainder of the coating is the organic resin.        2. The magnetic steel sheet with a semi-organic insulation coating according to the item 1, wherein the plate-like silica has an average particle size of 10 to 600 nm.        3. The magnetic steel sheet with a semi-organic insulation coating according to the item 1 or 2, wherein the plate-like silica has an aspect ratio (average length/average thickness ratio) of 2 to 400.        4. The magnetic steel sheet with a semi-organic insulation coating according to any one of the items 1 to 3, wherein the inorganic component further contains a B compound as a ratio in a dry coating of 0.1-5 mass % of B compound (converted at B2O3).        5. The magnetic steel sheet with a semi-organic insulation coating according to any one of the items 1 to 4, wherein the coating further contains not more than 30 mass % in total of one or more selected from a nitric acid compound (converted to NO3), a silane coupling agent (converted to a solid content) and a phosphorus compound (converted to P2O5) as a ratio in a dry coating.        
In other words, we provide a magnetic steel sheet having on its surface a semi-organic insulation coating comprising an inorganic component and an organic resin, wherein as the inorganic component is included                (1) a Zr compound and an Si compound containing a plate-like silica as a ratio in a dry coating of 20-70 mass % of Zr compound (converted to ZrO2) and 10-50 mass % of Si compound containing plate-like silica (converted to SiO2);        (2) if necessary, a B compound as a ratio in a dry coating of 0.1-5 mass % of B compound (converted to B2O3); and        (3) if necessary, not more than 30 mass % in total of one or more selected from a nitric acid compound (converted to NO3), a silane coupling agent (converted to a solid content) and a phosphorus compound (converted to P2O5) as a ratio in a dry coating,                    the remainder of the coating is substantially the organic resin (i.e., with an allowance of containing impurities and known additives), and moreover, preferably,            the average particle size of the plate-like silica is 10 to 600 nm and/or the aspect ratio (average length/average thickness ratio) of the plate-like silica is 2 to 400.                        
Our magnetic steel sheets with a semi-organic insulation coating are excellent in various properties such as powdering resistance, scratch resistance, sticking resistance, TIG weldability and punchability, and has no deterioration of water resistance and corrosion resistance without containing a chromium compound, and also are excellent in the uniformity of coating appearance after annealing.